FIG. 8 is a configuration drawing to show an orthogonal-type gas laser in a related art. In the figure, numeral 1 denotes a laser oscillator, numeral 2 denotes a discharge electrode in the laser oscillator 1, numeral 3 denotes a gas circulation blower in the laser oscillator 1, numeral 4 denotes a partial reflecting mirror, numeral 5 denotes a total reflecting mirror, numeral 6 denotes a heat exchanger, numeral 7 denotes a cooling unit, numeral 8 denotes a power supply panel, numeral 9 denotes a control unit, numeral 10 denotes a laser medium, and numeral 11 denotes laser light taken out from the laser oscillator 1. The partial reflecting mirror 4 and the total reflecting mirror 5 make up a resonator 12. The cooling unit 7 cools the partial reflecting mirror 4, the total reflecting mirror 5, and the heat exchanger 6. A machine for generating discharge in the discharge electrode 2, a machine for controlling the gas circulation blower 3, a machine for producing a vacuum in the laser oscillator 1, and the like are placed in the power supply panel 8.
Next, the operation of the orthogonal-type gas laser in FIG. 8 will be discussed. The machine for controlling the gas circulation blower 3 in the power supply panel 8 is driven by a start signal from the control unit 9, whereby the gas circulation blower 3 is rotated and the laser medium 10 with which the laser oscillator 1 is filled, for example, CO2 gas in a carbon dioxide laser is circulated. In this state, if an output signal is given from the control unit 9, a high voltage is input to the discharge electrode 2 and the laser medium 10 is excited because of discharge. The excited laser medium 10 emits light and drops to the base level. The emitted light is reflected and amplified between the partial reflecting mirror 4 and the total reflecting mirror 5 making up the resonator 12. That is, some of the laser light is taken out to the outside from the partial reflecting mirror 4 and the remainder is further reflected on the total reflecting mirror 5 and is reflected and amplified repeatedly. The laser light 11 taken out to the outside is controlled so that the light corresponding to output of a command of the control unit 9 is taken out. The configuration in FIG. 8 is called three-axis orthogonal type because the three directions of the direction of the laser light 11, the discharge direction, and the direction in which the laser medium 10 flows between the discharge electrodes 2 are orthogonal to each other. The laser light 11 taken out from the laser oscillator 1 is transmitted to a laser beam machine, etc., and is used for working of cutting, welding, etc., measuring, etc.
FIG. 9 is a configuration drawing to show the positional relationship between reflecting mirrors and discharge electrodes in an orthogonal-type gas laser with a resonator configured for turning laser light by three total reflecting mirrors, disclosed in Japanese Patent Laid-Open No. 127773/1985. FIG. 9(a) is a sectional view of viewing laser oscillator from the optical axis direction of laser light 11. FIG. 9(b) is a sectional view of viewing laser oscillator from a direction orthogonal to the optical axis direction of the laser light 11; it shows a laser light path. In the figure, numeral 12 denotes a resonator, numeral 13 is a partial reflecting mirror, numerals 14 to 16 denote total reflecting mirrors, numerals 17 denote apertures placed in front of reflecting mirrors corresponding thereto and having a guide function of shape determination of beam mode and laser light amplification, and numeral 18 denotes a discharge space. The total reflecting mirrors 14 and 15 are placed in the laser light path between the partial reflecting mirror 13 and the total reflecting mirror 16 and the laser light reflected from the partial reflecting mirror 13 is turned three times by the total reflecting mirrors 14, 15, and 16 and then is returned on the same light path.
FIG. 10 is a drawing to show a gain distribution by discharge in orthogonal-type gas laser; it shows how the gain changes depending on the position in the direction in which the laser medium 10 flows. From FIG. 10, it is seen that the gain is higher downstream in the direction in which the laser medium 10 flows in the discharge area. Based on such a characteristic, the laser light path is also placed at the downstream end in the direction in which the laser medium 10 flows in the configuration in FIG. 9.
Next, the reason why the resonator 12 is configured for turning laser light by a plurality of reflecting mirrors as in FIG. 9 will be discussed based on theoretical expressions of lasing.
Laser output Wr is given by the following expression:Wr=η·(Wd−W0)  (1)where η is excitation efficiency, Wd is discharge input, and W0 is a lasing threshold value. The excitation efficiency η is given by the following expression:η=F·η0  (2)where F is a discharge space utilization factor and η0 is conversion efficiency of laser medium to light.
The lasing threshold value W0 in expression (1) is given by the following expression:W0=w0/m  (3)where w0 is a parameter derived from the loss of the whole resonator such as the transmissivity of the partial reflecting mirror forming a part of the resonator and m is the number of times laser light is returned.
From expression (1), it is seen that the higher the excitation efficiency η and the lower the lasing threshold value W0, the larger the laser output Wr, namely, the higher the conversion efficiency to laser light. From expressions (2) and (3), it is seen that the higher the discharge space utilization factor F, the higher the excitation efficiency η and the larger the number of times laser light is returned m, the lower the lasing threshold value W0 and therefore a high-efficiency orthogonal-type gas laser can be provided. Thus, the orthogonal-type gas laser with the resonator configured for turning laser light by a plurality of reflecting mirrors is used for the purpose of providing a compact orthogonal-type gas laser having high conversion efficiency to laser light.
Such high efficiency provided by the configuration of turning laser light by a plurality of reflecting mirrors is a characteristic phenomenon and cannot be realized until laser medium is excited by discharge while laser light is reciprocated more than once in the same discharge space. That is, it cannot be realized in the configuration in which only one optical axis exists in one laser tube like an axial-type gas laser, for example, disclosed in Japanese Utility Model Laid-Open No. 29969/1981.
The orthogonal-type gas laser has the configuration as shown above in FIG. 9 for enhancing the lasing efficiency of laser light; however, still higher efficiency is desired from the demand for energy saving in this day and age. Demand for a more compact orthogonal-type gas laser is increased from the viewpoint of saving space.
As the number of times laser light is turned is increased, efficiency can be made higher as described above, but it is difficult to further increase the number of times laser light is turned in the configuration in FIG. 9. The reason is that the spacing between the discharge electrodes is limited because of stable discharge generation and normally is 100 mm or less and it is difficult to place all optical axes at the above-mentioned downstream end from the limitations on placement of the reflecting mirrors and the structure of a holder for holding the reflecting mirrors. Further, the reason is that the shape symmetry of output laser light is degraded because of laser light overlap caused by turning laser light and directivity occurs in working using the output laser light, for example.